It has been maintained, and apparently With :1 great deal of truth, and several eminent men who have studied the sub ject deeply have coincided in the opinion, that Where an arch is constructed of several key-stones or vonssoirs, and retn,titi4 in equilibriu. its various parts being so as to came it to remain without change of position, that a catenary or funi cular curve will be thund to be wholly included somewh,re between its intrados and its extrados. Now, as a wooden bridge, formed on the principle of compression. either by a polygonal frame or with a timber arch, should be to conform as much to the principle of the arch as possible, so as to make it as strong as can be, with a uniform pressure at every point by which it may be equally sustained, without throwing an undue strain upon any of the joints or beams, it is well to make the distance between the intrados and extrados as great as can be; hence the advantage of a double arch of timbers, with the intermediate space tided in with judiciously anauged braces and diagonal pieces, by which the ribs are kept in their proper position without materially increasing the weight.
In the construction of wooden bridges it must be con stantly borne in mind that every piece of timber introduced should be placed, as fir as can be, so as to bear the strain which will be thrown upon it in the direction of its length; for the wore it inclines from the direction of the thrust to which it is opposed, the less resistance it will be capable of exciting, and its useful effect will decline in the ratio of the radius to the sine of the angle of its inclination. In addition to this, the tendency to fracture is much greater by a trans verse strain than by a force acting longitudinally by coin pressitin through the entire length of the been.
\\ lien it is required to join two or more pieces of timber end to end, so as to make them, together, sufficiently long for any required purpose, the joint made use of in that case is called it scarf This is made by cutting away a portion of the thickness of the wood in both pieces, so that they may fit into each other : the scarf is then made complete by bolting them well together with iron bolts, or with trenails, and putting iron straps round the joint where necessary. A scarf may be made by cutting away halt' the wood at the end of each piece, that so they may partially overlie or lap upon each other, or else, in a more advantageous way, by cutting away the wood in a slanting direction, at the same time making indentations or notches in•one piece which exactly coincide with similar projections cut near the end of the piece to be joined to it; such scarfs are usually from 18 inches to 3 feet long.
Teuon-and-mortise joints are used for uniting timbers at right angles to one another. The thickness is diminished en both sides at the end of a piece of wood, so as to leave an oblong projection, called a tenon, which is fitted into a termed a mortise, hollowed out of the piece to which it is to be united, and placed at about a third of the height from the under side; a wooden pin is frequently put through the mom. Use and tenon so as to prevent them from getting out of their places. A combination of a tenon-and-mortise joint with a scarf' is sometimes made for joining timbers halgituditmlly ; and it is found to add greatly to its strength. Scarf's in beams likely to undergo a transverse strain must always be carefully made; they should be very long and well notched, and this tenon-and-mortise joint added, the whole strongly bolted; and it is often a good precaution to put a plate of iron on the under and upper sides of the scarf.
A dove-tail joint is made by forming a tenon increasing in width towards the extremity of the wood, and having a narrow neck near the shoulder; this is fitted to a mortise made in the same shape, on another piece of wood, into ‘‘ inlet it is admitted laterally ; when the wood is hard, this is a good kind of joint, but the ends of the inverted wedges are liable to be broken off if their sides diverge too intich, and if they are made in too straight a direction, they are apt to shrink and thus become useless. This joint is, however, very suit able for ties when well made, and it is used .n cases where it is thought desirable to smeeure two horizontal pieces of wood more firmly together, which are joined to one another at au angle. In that case, a piece of wood is made to act as it tie, by being dove-tailed into both the timbers at some distance from the joint.
The ends of struts and braces are usually let into the tim-.
bers with which they are connected at the ends by mortise and-tenon joints; these joints are generally cut at the ends perpendicularly to their direction, and in this way resist the strains which they have to bear more effectually.
A king-post is a plincipal vertical timber, to which the centres of bays or girders are attached, and by which means, properly speaking, they should be sustained.
Queen-posts are similar to king-posts, but do not occupy so important a position in a piece of carpentry. A number of these may be introduced with great advantage; they should be well framed into the top and bottom timbers, either by halving or by mortise-and-tenon joints; and iron straps should, in must cases, be made use of to make them secure.